1. Field of Invention
The present invention relates to hybrid solar fossil fuel receivers and, in particular to hybrid sodium heat pipe receivers for dish/stirling systems in follow up to and based upon Provisional Application Ser. No. 60/222,875, filed Aug. 3, 2000, and claims the benefit of the priority filing date of said Provisional Application under 35 U.S.C. Section 119(e).
2. Description of the Related Art
Solar dish/sterling systems continue to receive strong interest in concentrating solar research programs, because of their demonstrated high efficiency for conversion of sunlight to electricity. Potential end users have indicated that to satisfy their requirements for continuous, reliable, and economical electricity, these systems will need to be hybridized. Hybridization adds a combustor and two heat exchangers to the existing concentrator, receiver, engine, and electrical system. This addition should cost less than $300/kW to compete with its diesel alternative. In addition to this economic challenge, there is the technical challenge of efficiently firing an engine at 700xc2x0 C. or more. This requires a well-designed primary heat exchanger as well as a carefully-integrated combustor and recuperator.
Over the past decade or so, a number of programs have addressed various aspects of these challenges. Most have used alkali-metal reflux receivers as the starting point. These receivers are popular because of their isothermal behavior. Their primary benefit is higher system efficiency, enabled by uniform temperature at the Stirling-engine heater heads. For hybrid systems, reflux receivers have a further benefit: they allow separate solar and fired heat-transfer surfaces, and therefore independent optimizations. Conceived nearly 20 years ago, Osborn, D. B., et al., xe2x80x9cSolar Power Converter with Pool Boiling Receiver and Integral Heat Exchanger,xe2x80x9d U.S. Pat. No. 4,335,578, Jun. 22, 1982, alkali-metal reflux receivers have been under intensive development since about 1987. Andraka, C. E., et al., xe2x80x9cReflux Heat-Pipe Receivers for Dish Electric Systems,xe2x80x9d Proceedings of the 22nd Intersociety Energy Conversion Engineering Conference, Philadelphia, Pa., 1987; Diver, R. B., et al., xe2x80x9cSolar Test of an Integrated Sodium Reflux Heat Pipe Receiver/Reactor for Thermochemical Energy Transport,xe2x80x9d Journal of Solar Energy, 1990; Andraka, C. E., et al., xe2x80x9cTesting of Stirling Engine Solar Reflux Receivers,xe2x80x9d Proceedings of the 28th Intersociety Energy Conversion Engineering Conference, Atlanta, Ga., 1993; and Adkins, D. R., et al., xe2x80x9cHeat Pipe Solar Receiver Development Activities at Sandia National Laboratories,xe2x80x9d Proceedings of the Renewable and Advanced Energy Conference, Maui, Hi., 1999.
In 1991, the Institute for Physics and Power Engineering (IPPE, Obninsk, Russia) reported on several sodium and NaK heat-pipe designs used to transmit power to Stirling engines. Gonnov, I. V., et al., xe2x80x9cDesign and Testing of Heat Exchangers with Liquid Metal Heat Pipes for Stirling Engines,xe2x80x9d Proceedings of the 26h Intersociety Energy Conversion Engineering Conference, Boston, Mass., 1991. The IPPE designs included gas-fired and solar-heated versions, all with screen wicks. The gas-fired surfaces were elaborate high-parts-count assemblies. Nominally-isothermal operation was demonstrated with metal-vapor temperatures up to 750xc2x0 C. and electrical output up to 4 kWe. The issues of simultaneous gas and solar (hybrid) operation were not addressed.
Also in 1991, The German Aerospace Research Establishment (DLR) Institute for Technical Thermodynamics (Stuttgart, Germany) reported on their development of a sodium heat pipe receiver with screen wicks, demonstrating transport of 32 kWt at 780xc2x0 C. Laing, Doerte, et al., xe2x80x9cSodium Heat Pipe Solar Receiver for a SPS V-160 Stirling Engine: Development, Laboratory and On-Sun Test results,xe2x80x9d Proceedings of the 26 th Intersociety Energy Conversion Engineering Conference, Boston, Mass., 1991. Since then, the DLR has continued the development of its design. Laing, D., et al., xe2x80x9cSecond Generation Sodium Heat Pipe Receiver for a USAB V-160 Stirling Engine: Evaluation of On-Sun Test Results Using the Proposed IEA Guidelines and Analysis of Heat Pipe Damage,xe2x80x9d Journal of Solar Energy Engineering, November, 1997, and most recently, reported on first- and second-generation hybrid designs. Laing, D., et al., xe2x80x9cDesign and Test Results of First and Second Generation Hybrid Sodium Heat Pipe Receivers for Dish/Stirling Systems,xe2x80x9d Proceedings of the ASME International Solar Energy Conference, Albuquerque, N.Mex., 1998. The DLR hybrids are completely-integrated systems, including a Stirling engine, screen-wick heat-pipe receiver with separate solar and gas-fired surfaces, a natural-gas combustor, a brazed-fin primary heat exchanger, and a recuperator. The first system used a diffusion gas-swirl burner. It was operated for more than 60 hours, with xe2x80x9cvery acceptablexe2x80x9d behavior. The DLR has presented results showing burner operation between about 8 and 22.8 kWt, sodium vapor temperatures up to 790xc2x0 C., system efficiencies up to 20% (gas only, with the aperture plugged) and combustor efficiencies up to 90%. The second DLR hybrid represents a significant re-design. It uses a lean pre-mix combustion system, chosen to reduce exhaust emissions. The engine heater tubes are relocated to simplify manufacturing.
In 1994, Thermacore reported on its first hybrid heat-pipe receiver, developed for the Cummins Power Generation 7.5 kWe dish/Stirling system. Hartenstine, J. R., et al., xe2x80x9cDevelopment of a Solar and Gas-Fired Heat Pipe Receiver for the Cummins Power Generation 7.5 kWe Dish/Stirling System,xe2x80x9d Proceedings of the 29th Intersociety Energy Conversion Engineering Conference, Washington, D.C., 1994. Thermacore""s first system included a sodium heat-pipe receiver, separate solar and gas-fired surfaces, a natural-gas combustor, and an integrated recuperator. It featured nickel-powder wicks, fins milled from the heat-pipe wall, and nozzle-mixing burners. Test results (not reported in the literature) led to a second design that uses pre-mixed metal-matrix burners and circular-finned secondary heat pipes to supply heat to the primary heat-pipe solar receiver. It is believed that this system was tested successfully, although, once again, the test results are not reported in the literature.
In 1995, Stirling Technology Company (STC) reported on its development of a hybrid 10 kWt NaK pool-boiler receiver. Noble, J. E., et al., xe2x80x9cTest Results from a 10 kWt Solar/Natural Gas Hybrid Pool Boiler Receiver,xe2x80x9d Proceedings of the 4th ASME/JSME Solar Engineering Joint Conference., Maui, Hi., 1995. The system comprises a NaK pool boiler, separate solar and gas-fired surfaces, a natural-gas combustor, and a stand alone recuperator. The burner was a pre-mixed metal matrix type, delivering heat radiatively and convectively to the pool-boiler wall. The system was thermally loaded with a water-cooled gas-gap calorimeter. Tests were carried out with lamp heating at STC, and later with solar heating at the High Flux Solar Furnace at National Renewable Energy Laboratory (NREL). Full hybrid operation at nominally 700xc2x0 C. was demonstrated during simulated natural cloud transients, with burner power varying by 2:1.
In 1995, our nascent hybrid receiver efforts were combined to develop a 75-kWt hybrid reflux receiver, with emphasis on manufacturability, cost, and lifetime. Using a ⅙th-scale gas-fired sodium heat pipe, the initial step was to select a candidate burner type and candidate gas fired surface configuration. In 1997, we reported on our study of the applicability of premixed metal-matrix radiant burner technology to hybrid systems. Bohn, M. S., xe2x80x9cApplication of Radiant Burner Technology to Hybrid Dish/Stirling Systems,xe2x80x9d ASME International Solar Energy Conference, Washington, D.C., 1997.
However none of the foregoing art enables a fully-integrated system, including a burner, pin-fin primary heat exchanger, recuperator, solar absorber, and sodium heat pipe, which is characterized in design to avoid pre-ignition, while attaining robust heat-pipe performance, and long life of the burner matrix, recuperator, and flue-gas seals.
It is therefor an object of the present invention to provide a fully-integrated hybrid-sodium heat-pipe receiver system, including a burner, pin-fin primary heat exchanger, recuperator, solar absorber, and sodium heat pipe.
It is another object of the present invention to provide a fully-integrated hybrid receiver system, which is characterized in design to avoid pre-ignition, while attaining robust heat-pipe performance, and long life of the burner matrix, recuperator, and flue-gas seals.
It is a yet another object of the present invention to provide a fully-integrated hybrid receiver system for efficiently at 700xc2x0 C., and more particularly 750xc2x0 C. sodium vapor temperature.
It is yet another object of the invention to provide a hybrid-receiver having a 68 kWt solar, gas, or combined throughput.
It is yet another object of the invention to provide a compact metal-matrix radiant burner for use in a hybrid systems solar receiver characterized by low air emissions, consistent with existing and anticipated regulations, but which is not susceptible to pre-ignition when fired using a premixed fossil fuel/air mixture.
It is a further object of the invention to provide a combustion system capable of operation with a fuel/air mixture preheated to 640xc2x0 C. without pre-ignition.
Additional advantages of the present invention will be set forth in part in the description that follows and in part will be obvious from that description or can be learned from practice of the invention. The advantages of the invention can be realized and obtained by the apparatus particularly pointed out in the appended claims.
Briefly, to overcome the problems of the prior art and in accordance with the purpose of the invention, as embodied and broadly described herein, a hybrid high-temperature solar receiver of the present invention comprises a solar heat-pipe-receiver including a front dome having a solar absorber surface for receiving concentrated solar energy, a heat pipe wick, a rear dome, a sidewall joining the front and the rear dome, a vapor and a return liquid tube connecting to an engine, and a fossil fuel fired combustion system in radial integration with the sidewall for simultaneous operation with the solar heat pipe receiver, the combustion system comprising an air and fuel pre-mixer, an outer cooling jacket for tangentially introducing and cooling the mixture, a recuperator for preheating the mixture, a burner plenum having an inner and an outer wall, a porous cylindrical metal matrix burner firing radially inward facing a sodium vapor sink, the mixture ignited downstream of the matrix forming combustion products, an exhaust plenum, a fossil-fuel heat-input surface having an outer surface covered with a pin-fin array, the combustion products flowing through the array to give up additional heat to the receiver, and an inner surface covered with an extension of the heat-pipe wick, a pin-fin shroud sealed to the burner and exhaust plenums, an end seal, a flue-gas diversion tube and a flue-gas valve for use at off-design conditions to limit the temperature of the pre-heated air and fuel mixture, preventing pre-ignition.